1. Field of the Invention
The present invention generally relates to an image forming device wherein an image is recorded to a sheet such as a paper or an OHP (Overhead Projector) film by using an electrophotographic method, for example, such as a copier, a printer, a facsimile device, or a complex machine consisting of the copier, the printer, and the facsimile. Furthermore, the present invention generally relates to a sheet carrier, by which the sheet is carried along a sheet carriage path, of a sheet usage device such as the above mentioned image forming device.
2. Description of the Related Art
Conventionally, an electrophotographic type image forming device having a sheet carrier described in Japanese Laid-Open Patent Application, No. 11-59965, is used. As shown in FIG. 1, for example, in this kind of sheet carrier, a sheet S sent from a sheet loading part 1 is carried to a drum-shaped photoconductor body 3 via a sheet carriage path 2. Then, a toner image formed on the photoconductor body 3 is transferred to the sheet S by a transcription roller 4, with the rotation of the photoconductor body 3.
In this sheet carrier, a first sheet carriage part 5, a second sheet carriage part 6, a third sheet carriage part 7, and a fourth sheet carriage part 8 are provided in sequence from the sheet loading part 1 to a downstream side along the sheet carriage path 2. Detection parts a, b, and c are provided at a just downstream positions from the first sheet carriage part 5, the second sheet carriage part 6, and the third sheet carriage part 7, respectively. A detection part d is provided at a just upstream position from the fourth sheet carriage part 8.
The first sheet carriage part 5 includes a pick up roller 5a forming a sheet feeding part, a feed roller 5b forming a separation part of FRR separation type, and a reverse roller 5c. 
If a sheet feeding signal is on, the pick up roller 5a descends and rotates so that the sheets S loaded at the sheet loading part 1 are picked up by the pick up roller 5a from an upper side in sequence. Thee sheets S are separated and sent by the feed roller 5b and the reverse roller 5c. The sheet S is carried by a pair of the carriage rollers 6a and 6b of the second sheet carriage part 6 and a pair of carriage rollers 7a and 7b of the third sheet carriage part 7 via the sheet carriage path 2. A head end part of the sheet S is carried into resist rollers 8a and 8b of the fourth sheet carriage part 8 so as to be stopped and thereby a skew is corrected. After that, the resist rollers 8a and 8b are started rotating in timing with the toner image of the photoconductor body 3 so that the image position is adjusted and the sheet S is sent to a lower side of the photoconductor body 3.
When the head end of the carried sheet S is detected by the first detection part a, the rotation of the pick up roller 5a is stopped. When the head end of the carried sheet S is detected by the second detection part b, the rotations of the feed roller 5b and reverse roller 5c are stopped. When the head end of the carried sheet S is detected by the third detection part c, image writing to the photoconductor body 3 starts. The rotations of the carriage rollers 6a, 6b, 7a and 7b are stopped, after the head end is detected by the fourth detection part d and a designated time (time interval) passes.
Thus, the head end of the carried sheet S is detected so that the rotations of the pick up roller 5a, the feed back roller 5b, the reverse roller 5c, and the carriage rollers 6a, 6b, 7a, and 7b are stopped and the image writing starts. In addition, the head end part of the carried sheet S is carried into resist rollers 8a and 8b of the fourth sheet carriage part 8 so as to be stopped. Hence, when the image is recorded to the sheet S continuously, it is necessary to make a proper space between the sheets in front and behind.
However, due to increase in demand for providing image forming devices having high performance in order to be distinguishable from other companies' products, it is desired to increase the number of sheets where the image is recorded, per unit time. Thus, if a speed for recording the image is increased by making the performance of a motor high, it causes an increase of cost and noise, and reduces durability. If the space between the sheets in front and behind is made as small as possible, it is possible to increase the number of sheets where the image is recorded, per unit time, without making the performance of the motor high.
However, in the sheet carrier where such an FRR separation method is applied, the sheets S loaded at the sheet loading part 1 are picked up by the pick up roller 5a from the upper side in sequence. The sheets S are separated and sent by the feed roller 5b and the reverse roller 5c. Therefore, when the front sheet S is sent, the position of the head end of the rear sheet S is made non-uniform between the head end position p of the loaded sheet and the separation nip position q of the separation part.
Because of this, if the space between the sheets S in front and behind is made as small as possible, at the greatest, the front and rear sheets are carried under a state where the rear end of the front sheet S and the head end of the rear sheet S are overlapped by a length of space h between the head end position p of the loaded sheet and the separation nip position q of the separation part.
Carriage of the sheets S under a state where the front sheet and rear sheet are overlapped causes the following problems. First, it is difficult to detect the head end of the rear sheet S and therefore it is difficult to stop the rotations of the pick up roller 5a, the feed roller 5b, the reverse roller 5c, and the carriage rollers 6a, 6b, 7a, and 7b, and start the image recording. Secondly, it is difficult to stop the sheet S in a state where the head end part of the sheet S is carried into resist rollers 8a and 8b of the fourth sheet carriage part 8.